Aircraft engines generate significant sound during operation. Excessive engine sound is generally undesirable since excessive sound may disturb surrounding communities.
Conventional subsonic civil aircraft designs commonly feature engines placed underneath the aircraft's wings. The sound pressure level produced by the engines, herein generally referred to as “engine noise,” and particularly the engine noise generated during takeoff and landing, travels largely unabated to surrounding communities. For under-wing engine installations this noise is amplified by the lower surface of the wing because the portion of the sound produced by the engines that would otherwise radiate upward is reflected downward off of the lower surface of the wing. Furthermore, the engine exhaust plume may interact with the wing's trailing edge, especially during landing and/or takeoff when trailing-edge flaps may be extended further into the engine exhaust plume. Both the lower wing surface reflection and the engine exhaust plume interaction with the wing's trailing edge add to the overall noise heard below the aircraft.
Further exasperating the community noise problem, transportation and traffic planners frequently call for increased dependence on regional passenger air transportation to serve smaller regional airports that surround major international airport hubs. Regional air transportation creates an increased risk of community environment intrusion since regional air transportation operations are often confined within lower speed regimes that result in longer exposure times at lower altitude ceilings and in closer proximity to surrounding communities. Furthermore, the surrounding communities have become increasingly sensitive to extended exposures to aircraft noise.
Many approaches to reducing community noise primarily focus on the propulsion system itself and the design of the engine nacelle. For example, many attempts to reduce engine noise have resulted in higher bypass ratio turbofan engines, the use of negatively scarfed inlets, and the use of chevrons. While technological improvements in engines have resulted in a gradual reduction of engine noise over time, further reductions in the amount of engine noise reaching surrounding communities are desired.
A few examples exist that purposefully exploit engine installation as a primary noise reduction feature. In particular, in a small number of cases engines have been installed above the wings to yield some reduction in downward, forward, and/or sideline emanating noise via the principle of shielding. For example, U.S. patent application Ser. Nos. 11/307,271, 11/612,594, and 29/247,058 assigned to The Boeing Company describe aircraft configurations where the engine is positioned above the wing. Other examples of aircraft having engines positioned higher than the wing (in many cases for reasons other than noise) include the VFW-614, the Honda Jet, the Beriev Be-200, the Boeing 717, and the family of Gulfstream business jets. However, even when engines are located higher than the wings, these aircraft do not have wing planforms tailored specifically to impede the downward travel of sound due to the absence of a tailored planform surface that covers a substantial area of the downward sound propagation path. Furthermore, merely placing the engine above the wing to increase the amount of effective acoustic shielding typically also results in corresponding increases in induced drag, interference drag, and skin friction or parasite drag scaling with wetted area. Therefore, systems and methods are needed for reducing the engine noise directed toward surrounding communities without significantly impacting the aircraft's lift and drag characteristics in a negative way.
Furthermore, while “flying wing” type aircraft often are configured with the engines above the wing/fuselage portion of the flying wing, systems and methods are needed for reducing the engine community noise on a more conventional type of aircraft where two generally high aspect ratio main wings are coupled to the fuselage and where the horizontal stabilizer is separate and distinct from the two main wings.